This invention relates to a switch for detecting shift positions of an automatic transmission.
In general, a conventional shift position detecting switch for automatic transmissions has contact points which are independently provided for respective shift positions and which are changed over to establish or cancel connection to a single load at each position.
The construction of this type of switch will be described below with reference to FIGS. 1 and 2.
FIG. 1 shows the arrangement of a six-position switch which is an example of this type of conventional shift position detecting switch for automatic transmissions.
As shown in FIG. 1, on a terminal plate 1 made of a resin are formed first and second rows of stationary contact pieces which extend circumferentially about a pivot support shaft 3 of a movable contact piece 2 at inner and outer positions, respectively. The first row consists of a contact piece R.sub.B connected to a power source, and the second row consists of a contact piece P.sub.L to which a parking display (load) is connected, a contact piece R.sub.L to which a reverse display (load) is connected, a contact piece N.sub.L to which a neutral display (load) is connected, a contact piece D.sub.L to which a D-range display (load) is connected, a contact piece 2.sub.L to which a second-range display (load) is connected, and a contact piece L.sub.L to which a L-range display (load) is connected. That is, these stationary contact pieces are mounted in the terminal plate 1 by insert molding with a resin, and the movable contact piece 2 can be moved over the terminal plate 1 in a linked relationship with the shifting over of the shift positions of the automatic transmission so that it is connected to or disconnected from a contact point on the second-row contact piece with respect to each shift position. There is a desired arrangement of on-off states of the contact points of the second-row stationary contact pieces relative to the stationary contact piece R.sub.B. FIG. 2 shows combinations of on and off states at respective positions.
The switching on/off operation at each position will be described below.
Firstly, when the movable contact piece 2 is placed at a position P (parking position) by being moved in a linked relationship with the shifting to the corresponding shift position of the automatic transmission, only the contact point P.sub.L becomes on while all the contact points R.sub.L, N.sub.L, D.sub.L, 2.sub.L and L.sub.L become off.
When the movable contact piece 2 is placed at a position R (reverse position), only the contact point R.sub.L becomes on while all the contact points P.sub.L, N.sub.L, D.sub.L, 2.sub.L and L.sub.L become off.
When the movable contact piece 2 is placed at a position N (neutral position), only the contact point N.sub.L becomes on while all the contact points P.sub.L, R.sub.L, D.sub.L, 2.sub.L and L.sub.L become off.
When the movable contact piece 2 is placed at a D-range position, only the contact point D.sub.L becomes on while all the contact points P.sub.L, R.sub.L, N.sub.L, 2.sub.L and L.sub.L become off.
When the movable contact piece 2 is placed at a second-range position, only the contact point 2.sub.L becomes on while all the contact points P.sub.L, R.sub.L, N.sub.L, D.sub.L and L.sub.L become off.
When the movable contact piece 2 is placed at a L-range position, only the contact point L.sub.L becomes on while all the contact points P.sub.L, R.sub.L, N.sub.L, D.sub.L and 2.sub.L become off.
In the above-described system, however, each shift position is ascertained from switching of the corresponding one of the contact points. If a certain abnormality takes place, it is impossible for a control unit to detect this abnormality and, hence, to perform fail-safe control.